Some embodiments relate to a method and a system allowing moving tags to be discriminated from stationary tags, which can be used in the field of tracking merchandise or objects provided with RFID tags.
Geolocating merchandise by providing it with RFID tags that can be interrogated remotely by reading stations is known. The RFID tags can be read at various stages of the process of manufacturing or delivering the merchandise. They are, for example, read upon exiting the manufacturing unit in order to determine the merchandise that exits this unit and upon arrival in the warehouse of a client in order to determine the merchandise that is received by the client.
This merchandise is generally stacked in boxes and the boxes are moved via conveyors in the manufacturing unit or the receiving warehouse. The identification of the incoming or outgoing merchandise is carried out while it is present on the conveyor and is in motion. The difficulty is thus that of a reading station positioned near the conveyor only reading the moving tags present on the conveyor and not the tags possibly present in the surrounding environment of the conveyor. Indeed, since the signals used for reading the tags are UHF signals, it is very difficult to limit the reading zone to a well-defined volume around the conveyor because of the reflection of the UHF signals on walls of the surrounding environment and the generation of multipaths. The various paths of the response signals of a tag can thus add up and make it seem that this tag is in the reading zone. As a result, certain stationary tags distant from the conveyor can be read as being present on the conveyor.
In order to reduce these errors, it is possible to make the merchandise present on the conveyor pass through a metal tunnel and place the antennas of the reading station inside this tunnel. However, errors persist because the UHF signals can enter and exit the tunnel through its two ends.
The only known effective way that allows only the tags moving on the conveyor to be read is to set up a completely free (not containing tags or walls) security zone of approximately 50 m2 around the reading station, which requires having space.
Another problem is also that of being able to discriminate between the tags present in a box and other tags present in other boxes positioned downstream or upstream on the conveyor. A simple solution involves spacing apart the boxes on the conveyor, with a space of approximately 10 meters between two consecutive boxes, which is not at all acceptable in terms of conveying performance.
To solve this problem, international application WO2009/055839, filed by the present applicant, proposed temporally analysing the RSSI (Received Signal Strength Indication) level of the response signals of the tags in order to determine the moving tags and the stationary tags. It was noted that the moving tags each had an RSSI peak when passing in front of a reading station, whereas the RSSI level of the stationary tags was rather stable. Moreover, the moving tags having simultaneous RSSI peaks are the tags contained in the same box. This detection via temporal analysis of the RSSI levels of the tags allowed the distance between two consecutive boxes to be reduced and changed from 10 meters to 2 meters. However, in certain situations, stationary tags can have an RSSI peak and be counted as moving tags. Indeed, certain changes in the environment (a door that is opened, a person that moves, . . . ) can create, at a given instant, constructive interference (addition of a plurality of paths) that thus causes an RSSI peak. The only solution for preventing this thus remains the creation of the 50 m2 security zone.